1. Field of the Invention
This invention relates to coding.
2. Description of Prior Art
Modems communicate digital data over an analog communication medium by mapping the data onto analog signals. Some of today's sophisticated modems communicate digital information by mapping the data onto analog signals and, thereafter, modulating an analog carrier with those signals. Typically, a collection of bits are combined to form a two-dimensional symbol, the symbol components are converted to analog form, and the analog-form components respectively modulate a carrier and a quadrature replica of the carrier. The two modulation products are added and filtered, and the result is applied to the transmission medium. A remote modem receives the signal, samples it, identifies the magnitudes and phases of the analog samples, converts the samples to symbols, and finally recovers the original bits of data. What these modems do, in effect, is encode the digital signals onto a two dimensional symbol constellation that is modulated onto a carrier.
When the telecommunication network is completely analog the primary sources of error are signal echoes from imperfect hybrids and other discontinuities in the transmission lines, as well as noise from a myriad of unknown sources. When the telecommunication network includes digital links, where the analog data is samples and quantized, an additional noise source is introduced that is rooted in the sampling and quantization that occurs at the network.
One might believe that the additional noise caused by the quantization in the network would lower the maximum data rate that may be achieved, but a copending application, Ser. No. 07/963,539, filed Oct. 20, 1992, now U.S. Pat. No. 5,394,437, achieves actually higher data rates than those that can be achieved by conventional modems. The higher rate is realized by synchronizing the sampling in the new modem to the sampling which takes place in the .mu.-law codec that is in the network, and by arranging for the signal levels of the modem to coincide with the quantization levels of the .mu.-law codec. Basically, the modem of the 07/963,539 application insures that no error signal is created by the sampling process in the network.
While the improved modem of the '539 application solves the quantization problem created by the digital network's codec, a difficulty still arises from echoes and noise that are unavoidably introduced into the signal just prior to the network's quantization. This problem is overcome with a modem that encodes the symbols sent to the central office so that the correct symbols are decoded in spite of errors caused by the interaction of noise with echos. The challenge, however, is to encode the signal in an efficient manner.